The calibration of color displays, such as cathode ray tube (CRT) displays, has conventionally been provided by software in the personal computer coupled to the display for enabling a user to manually modify characteristics of color displays, or select predefined color transformation curves. Often, such calibration is a part of typical Desktop Publishing Software. Calibration may be improved by the use of a sensor for measuring color from a display, which can then enable automatic calibration with or without user intervention. An example of an automatic calibration system is shown in FIG. 1. FIG. 1 shows a video display 10 with cowel 11 and sensor 12 as generally described in U.S. Pat. No. 6,043,909 and U.S. patent application Ser. No. 09/139,498, integrated with a host computer's graphics display section 14 and a display controller for controlling display 10 function (e.g., brightness, contrast). Sensor 12 is called “lumeter” in application Ser. No. 09/139,498. Display 10 is shown in cross section as viewed from the side. An upper, heavy black line may represent a mounting arm coupled to the display which supports a sensor and may support a cowel. The cowel helps to shield the screen 13 of the display from ambient illumination and the sensor enables highly automatic (and even unattended) calibration. A circumferential cowel provides a “black trap” or surface which prevents any light from reflecting specularly off the screen of the monitor and into the sensor housing—in other words, the lower flange of the cowel provides better shielding from stray light and ensures that the sensor sees only light which is diffusely reflected from the screen. The latter is important to automatic measurement.
Many high performance displays have embedded microprocessors 15 which provides a digital controller for controlling the functions of the display. For example, it is possible to modify the gains in the R, G and B channels by sending signals from the host computer to the processor in the display. In some situations, the sensor 12 communicates directly with the display's processor, rather than, or in addition to, a host computer.
In displays which employ cathode ray tubes (CRTs) the gains in the R, G and B channels affect the voltages on the electron guns and thence the amount of light emitted by phosphors in response to electron bombardment. By adjusting analog gains and offsets, one can exert control over the tone reproduction and neutral balance of the display. When this method of control is available, it is preferred, but it requires knowledge of the control codes, which vary from one make of display monitor to another. The system of FIG. 1 may be adapted to interact with analogous controls in newer “digital displays.”
The operation of Graphics Display Section (software and hardware) of the host computer is illustrated in block 14. Device-independent color image data are fetched from RAM in the host computer and introduced to the graphics display section. The next two steps, namely 1) conversion in a 3×3 matrix (which handles the color mixture problem) and 2) processing of R, G and B data each through its own Look Up Table (LUT) can be combined in a color translator (which may be in International Color Consortium (ICC) or other standard format) for the video display. Channel-separate LUTs can be positioned both before and after the mixing matrix, as detailed in related U.S. Pat. No. 6,043,909 and U.S. patent application Ser. No. 09/139,498. The LUTs may provide a means of influencing neutral balance and tone reproduction. White-balance or neutral-balance is an adjustment to the display system that maps a neutral color represented by numbers in the computer, to a particular displayed color. In other words, it adjusts the balance between red, green and blue channels of the physical display in order to realize color coordinates of a particular, perceived white. Tone reproduction curves (TRC's) define the orderly progression of relative lightness values for the points in an image. Differently shaped curves mean that neighboring image points will be more or less similar in lightness. Tone reproduction is closely related to a property of displays known as gamma.
Processed data are stored in video RAM and are scanned repeatedly for transfer to the display. The LUTs and the DACs (Digital to Analog Converters) often limit the quality of the system. LUTs are often 8-bits in and 8-bits out, providing resolution that may not be acceptable in some applications. Therefore, although modifications to neutral balance and Tone Reproduction Curves (TRCs) can be effected in the LUTs, it is preferable to make most of the changes in the gains and offsets of the display's RGB channels, if possible, and to reserve the LUTs for “fine tuning.”
A generalized node of a Virtual Proofing network as described in U.S. Pat. No. 6,043,909 and U.S. patent application Ser. No. 09/139,498 is shown in FIG. 2. A nodal computer 20 receives calibration data (preferably in device independent coordinates which are preferably based on the Standard Observer) from (a) device(s) 22 which can measure a video display 21 and hard copy 23. (One device may be capable of all the necessary measurements or more specialized devices may handle different tasks.) The computer 20 processes the data and 1) modifies shared constituents of the Virtual Proof to be communicated through the network link 25 to other nodes, and 2) exercises appropriate control over the rendering devices (21 and 23) at the node.
Visual calibration may be performed for a display. For example, in U.S. Pat. No. 5,638,117, a card or reflector is used in visual calibration of a display. The card is used as a calibration reference by the user with respect to images displayed on the display to enable visual calibration. However, such calibration is not provided for as part of a virtual proof to enable other color devices in a network, which may be remote from each other, to render the same color.
Further, other approaches to assuring calibration of displays provides for locking display controls. For example, a “Color Sentry” feature for locking controls is described in U.S. Pat. No. 5,739,809. However, locking controls do not provide the capability for insuring that the controls are properly set for linear operation of the display.